In the golf ball art, ionomer resins which are metal ion-neutralized ethylene-(meth)acrylic acid copolymers have been widely used as the resin component in the cover outermost layer of solid golf balls and certain wound golf balls, because of their hitting durability and cut resistance. However, the ionomer resins have a higher hardness than the balata rubber conventionally used as the cover material. Thus golf balls using ionomer resins as the cover material are hard in hitting feel and inferior in spin control on iron shots, as compared with balata cover golf balls.
To overcome these problems, Egashira et al., U.S. Pat. No. 5,559,188 proposed the blending of a thermoplastic elastomer having a crystalline polyethylene block with an ionomer resin for rendering the ionomer resin flexible. According to this patent wherein a cover outermost layer is formed of a composition comprising an ionomer resin and a specific amount of a thermoplastic elastomer having a crystalline polyethylene block, the golf ball has a soft pleasant hitting feel, a minimized decline of repulsion or rebound, and resistance to scuffing upon iron shots.
However, simple blending of the ionomer resin with the thermoplastic elastomer often suffers from incompatibility therebetween. If a composition which has not been fully homogenized is used and molded into the cover outermost layer, there is a possibility that the layer be fluffed and ridged during grinding after molding, or the golf ball be reduced in rebound. It is described that the addition of a carboxyl or epoxy-modified olefin polymer as a third component is effective for improving the compatibility. The addition of such a third component, however, is impractical because the fluidity of the resin composition can be remarkably reduced to interfere with injection molding.
Ichikawa et al., U.S. Pat. No. 5,872,185 discloses to use a blend of an ionomer resin with a thermoplastic elastomer having a crystalline polyethylene block as an intermediate layer of a three-piece golf ball for improving the rebound and impact durability. Analogous problems can arise in this case too. When simple blends of the ionomer resin with the thermoplastic elastomer are employed in the intermediate layer, there is left a room for further improvement in the rebound of the golf ball.